A description of one such furnace will be of interest. It consists of a long rectangular building of brickwork bound together with steel framework. Inside it is divided up into low chambers, the roof of each forming the floor of the one above.
At intervals along its length mighty shafts of iron pass up from underneath right through all the floors, emerging finally above the topmost, while along underneath the furnace there runs a shaft the action
of which turns the vertical shafts slowly round and round.
Attached to the vertical shafts are long strong arms of iron, one arm to each floor, and upon the arms are placed rabbles, as they are termed, pieces of iron shod sometimes with fireclay, resembling most of any familiar objects a small ploughshare.
As the arms slowly revolve, at the rate of once or twice per minute, the arms are carried round and round and the rabbles plough up and turn over and over the layer of ore lying upon the floor.
There are arms on the top of the furnace, too, sometimes, where the ore is first laid so that it may be dried by the heat escaping from the furnace beneath, an interesting example of economy effected by utilizing heat which would otherwise be wasted.
The whole of the furnace, from end to end and on every floor, is thus swept continually by the rotating arms with their dependent rabbles, and the latter are cunningly shaped so that they not only turn the ore over and over, but gradually pass it along the different floors or hearths. It is fed automatically by a mechanical feeder which pushes it on, a small quantity at a time, to the drying hearth on the top. Then the rabbles take charge of it and gradually pass it from the area swept by one shaft to that of the next until it has passed right along the top and has become thoroughly dried. Arrived there it falls through a hole on to the topmost hearth or floor, along which it travels by the same means but in the contrary direction until it again falls through a hole on to the top floor but one.
And so it goes on until at last, fully roasted, it falls from the bottom floor of the furnace into trucks or other provision for carrying it away.
Some kinds of ore require to be heated by means of gas which is generated in a "gas-producer" near by. In others, however, the sulphur in the ore acts as the fuel, and so the furnace, having been once started, can be kept up for long periods without the expenditure of any coal at all. Very little attention is needed by furnaces such as these, so that with no fuel to pay for and very little labour, they are extremely economical.
Owing to the great heat, too, the arms would stand a very good chance of getting melted were they not kept cool by a continual stream of water flowing through the shafts and arms. This furnishes a continual supply of hot water which is sometimes used for other purposes in the works.